The present invention relates to a device and method for verifying the proper operation of dynamometers. The inventive device and method have particular application to the field of automobile and engine testing.
Measurement of the exhaust emissions, fuel consumption, and performance of automobiles requires the accurate simulation of on-road operating conditions in a fixed location. Simulation of on-road operation is typically accomplished using a treadmill-like device called a chassis dynamometer.
A dynamometer, which applies a resistance (or xe2x80x9cloadxe2x80x9d) to the vehicle wheels as they turn one or more rollers, can be used to simulate a variety of driving conditions or testing scenarios including transient (i.e., xe2x80x9cstop and goxe2x80x9d) drive cycles. Dynamometer devices are described in U.S. Pat. Nos. 5,861,552, 5,450,748, 5,101,660, 6,044,696, which are hereby incorporated by reference in their entirety.
In recent years, the use of chassis dynamometers has been required in state-mandated testing programs for the identification and repair of vehicles with excessive emissions. This requirement has stimulated a large increase in the number of dynamometers in use and a corresponding need to assure their proper operation.
In emissions testing, transient drive cycles are employed to simulate the conditions observed in normal driving and are characterized by variations in vehicle speed and dynamometer load as a function of time. However, if the load is applied improperly, the test yields inaccurate results that may lead to falsely passing a defective vehicle in an emission test or falsely failing a vehicle that should have passed the test. Consequently, proper operation of the dynamometer is necessary to assure accurate simulation of vehicle operation and accurate measurement of emissions.
The key to proper dynamometer operation is application of the appropriate load at the proper vehicle speed over a given driving cycle. A variety of methods exist to accurately load dynamometers. These methods are described in U.S. Pat. Nos. 5,375,460, 5,385,042, 5,531,107, 4,466,294, 5,542,290, 5,657,227, and 5,375,461 which are hereby incorporated by reference in their entirety.
A dynamometer may apply two types of load known as inertia load and road load to the vehicle under test. The inertia load simulates the power required to accelerate the vehicle. The road load simulates the power required to overcome frictional load on the vehicle. The dynamometer roller(s) supply some of the inertial load with the remainder of the load typically provided by flywheels driven by the dynamometer roller(s). A device commonly referred to as a power absorption unit (PAU) provides the road load. The PAU is typically an electric motor, eddy current brake or a water pump (usually called a water brake). With adequate control, the PAU can also be used to provide the inertia load, thereby eliminating the need for heavy flywheels to simulate the weight of the vehicle being tested.
A dynamometer controller controls the amount of load provided by the PAU. The controller adjusts the power absorbed by the PAU based on the specified vehicle characteristics (e.g., weight) and the vehicle speed and acceleration rate being simulated. The relationship between vehicle characteristics (such as weight, frontal area, and aerodynamic drag coefficient), the speed-time profile being simulated, and the target value of power absorbed by the dynamometer can be calculated from standard equations representing the dynamics of linear motion. (See e.g., Automotive Handbook by Robert Bosch GmbH, 1996, available at www.sae.org). The proportion of total dynamometer load provided by the PAU is dependent on whether or not flywheels are used to supply the inertia load.
Although the description of how a dynamometer is controlled may seem relatively simple, in fact, accurate control of a dynamometer is difficult. Dynamometer controllers must compensate for inter alia PAU response times and must respond very quickly to changes in vehicle speed when the PAU is used to supply inertia load. The dynamometer must also be xe2x80x9ctunedxe2x80x9d so that the PAU load properly accounts for dynamometer-specific factors which may change over time as the dynamometer components age and wear. Proper tuning requires an independent means of measuring dynamometer performance. However, an appropriate test device or method to carry out such testing has, until relatively recently, not been available.
In fact, little has been done to assure proper dynamometer performance during an actual test without using a dynamometer tester. In the past, manufacturers made and sold dynamometers without providing customers with the ability to independently verify whether the dynamometer was operating properly. If problems were suspected with a particular dynamometer system (e.g., an anomalous test result, such as abnormally high vehicle failure rates, was obtained), the commonly used means to crudely check performance was to employ a xe2x80x9ccross-checkxe2x80x9d vehicle. In this test, a single cross-check vehicle was tested on several different systems to determine whether a statistically significant difference existed between emission levels produced with different dynamometers. However, this method is confounded by measurement errors in the vehicle exhaust sampling system and cannot identify the cause of the test discrepancy. Clearly, there is a need for a device and method to test the performance of dynamometers to verify their proper operation, and to assist in the diagnosis of problems if found.
One such test device was recently developed under contract to the California Bureau of Automotive Repair (BAR) (See e.g., BAR RFP Solicitation Package #95/96-001 xe2x80x9cSolicitation Package For the Selection of a Contractor to Assist the State in the Design and Development of Device(s) or System for Performing Certification Testing of Dynamometers and Dynamometer Controlsxe2x80x9d, Sep. 15, 1995, Department of Consumer Affairs, Bureau of Automotive Repair, 10240 Systems Parkway, Sacramento, Calif. 95827). The BAR test device quantifies dynamometer performance by measuring the accuracy with which a dynamometer performs transient loading simulation. The BAR test device operates by sitting on the dynamometer rollers like a car and controls both the speed and acceleration of the dynamometer while simultaneously measuring the dynamometer""s load using the BAR test device""s own load cell (which measures the torque from the dynamometer tester""s electric motor). The BAR test device uses an external fifth wheel to quantify speed and determines dynamometer performance during transient simulation by integrating the speed and load signals and comparing them to the theoretical loading for the selected vehicle characteristics. This device is further described in Society of Automotive Engineers (SAE) paper 970268 (xe2x80x9cDynoCalxe2x80x94A Chassis Dynamometer Calibratorxe2x80x9d, SAE International Congress and Exposition proceedings, Feb. 24-27, 1997, available at www.sae.org) which is hereby incorporated by reference.
The BAR test device identified a variety of problems with dynamometers that had never before been known or quantified. For example, the dynamometer control system, which was previously discussed, is often either under-damped or over-damped. Over-damped systems are slow to respond and reach the target load, whereas under-damped systems over-react to system inputs and behave erratically. In addition to dynamometer control problems, problems affecting vehicle loading accuracy were also readily apparent for the first time using the BAR test device. For example, during a typical vehicle test an independent computer instructs the dynamometer controller on which vehicle parameters to use. If this external computer selects the wrong parameters, the dynamometer will apply the wrong load even if the dynamometer is working correctly. This problem cannot be easily detected on transient tests without using a dynamometer tester such as the BAR test device.
In short, the development and use of the BAR test device made obvious the importance of dynamometer testing. To date, dynamometer testing using the BAR test device has focused on initial certification testing of dynamometers. However, in use testing is also important to ensure that equipment used in the field remains accurate. Such in-use quality assurance testing can only be done with a dynamometer tester. However, as set forth more fully below, the BAR test device is bulky and difficult to move. Thus, there is a need in the art for an improved and portable dynamometer test device and method of using same that can more accurately evaluate dynamometer performance in diverse locations.
The present invention is a test device and method for the evaluation of the performance of a dynamometer. The test device comprises a data acquisition system including a system to determine speed and to measure a signal from a load measuring device such as a load cell. Using custom software to automate data collection and calculation of the results (using the calculations described below) the system reads the load equivalent cell signal from either the dynamometer, the dynamometer""s controller, or the dynamometer test system, and then collects speed data from the dynamometer or a separate speed measurement device during a test. A xe2x80x9cload equivalent cell signalxe2x80x9d is understood to mean a signal which may be a voltage, a digital signal or comparable output that provides the dynamometer test system feedback as to what the actual dynamometer load was, and is referred to herein below as a xe2x80x9cload cell signal.xe2x80x9d The software enables the collected data to be analyzed to determine the accuracy with which the load is applied, as compared to what is theoretically required.
The test method comprises the steps of calibrating the inventive test device to accurately read the dynamometer""s load measuring device or load cell signal, calibrating the speed measurement device, determining the parasitic drag in the dynamometer, and then performing a test on the dynamometer with a vehicle while collecting speed and load data. If the calibration settings are already known, or the parasitic loss characteristics of the dynamometer are already known, then these portions of the method can be omitted and the information can be entered directly into the inventive device software. When the test on the vehicle is complete, a spreadsheet with the calculations described below is used to automate calculation of the actual load applied and the theoretical load that should have been applied. This load can then be compared to the theoretical load that should have been applied to the vehicle to determine the difference (if any) between the measured load and the theoretical load. The magnitude of the differential enables an operator to easily determine if the dynamometer is operating properly.
The inventive test device is an improvement over the BAR test device 20 (FIG. 1) in the simplicity of its use, convenience, versatility, and cost. First, the BAR test device is large and cumbersome, and weighs nearly 2000 lbs. It must be moved around in a specially built trailer 19 containing the control system 18, motor 14, and wheels 13, and must be unloaded with a winch 17 which is both time consuming and dangerous. Secondly, the BAR test device is complex and must be operated by a highly trained and experienced person in addition to at least one, if not two, assistants. In the hands of less skilled operators, the BAR test device can easily create misleading and inaccurate results. Furthermore, the operation of the BAR test device can itself introduce uncertainty into the test measurement. When positioned on a dynamometer, the BAR test device measures force applied at the surface of roller 16, through its tires 13, which sit on the dynamometer rollers 16 (FIG. 2). Consequently, slippage (xe2x80x9ctire lossesxe2x80x9d) can occur between the BAR tester""s tires 13 and the dynamometer rollers 16 that must be accounted for. Compensating for tire losses is especially complicated because these losses change with tire temperature, which varies depending on the test cycle. Thirdly, the BAR test device lacks versatility in that it has only one axle and therefore cannot test four wheel drive dynamometers that electronically couple the front and rear axles on the dynamometer. Without mechanical coupling, it is impossible to accurately characterize dynamometer loading with certainty by only measuring the load on one of the axles of a multi-axle dynamometer. In fact, in these cases, dynamometer manufacturers must include load measuring devices or load cells at each axle to properly control load. In addition, since the BAR test device measures load while riding on chassis dynamometer rollers, it cannot be used to evaluate engine dynamometers where there are no rollers. Finally, the BAR test device is prohibitively expensive. The cost of the BAR test device can easily exceed $500,000. Operational costs are also high due to the need for highly skilled operators.
In contrast, the inventive test device fits easily into a 4xe2x80x3xc3x9712xe2x80x3xc3x9714xe2x80x3 carrying case, and can be operated by a single person with minimal training. Since the inventive test device determines load directly from the dynamometer""s own load measuring device or load cell instead of at the surface of the dynamometer rollers as in the BAR test device (as described above), the inventive test device is not affected by tire losses, thereby eliminating a major source of error. The inventive test device can also be used to test chassis dynamometers where sets of rollers are not physically coupled together as in electronically coupled four-wheel-drive dynamometers and split roller dynamometers that are not mechanically coupled side-to-side and which both utilize multiple power absorbers and load measuring devices or load cells. Furthermore, the inventive test device can be used to test engine dynamometers where there are no rollers and the engine is connected directly to the dynamometer. Lastly, the inventive test device can also be set up quickly and easily, with a cost less than one-tenth that of the BAR test device.
The inventive test device differs from the BAR test device in several ways. First of all, the BAR test device shown in FIGS. 1 and 2 uses an electric motor 14 to drive the dynamometer whereas the inventive test device uses any available automobile to drive the dynamometer. The electric motor 14 in the BAR test device accounts for the majority of its weight and inconvenience. The second difference between the BAR test device and the inventive test device, is that the BAR test device uses load measuring devices (such as load cells) 15 contained within it to read load generated by the electric motor 14 that is transmitted through tires 13 to the rollers 16, whereas the inventive test device uses the dynamometer""s own load measuring device (such as a load cell; 5 in FIG. 3) to measure load. Thus, the BAR test device measures total dynamometer load, whereas the inventive test device measures only load at the dynamometer load measuring device or load cell (which usually excludes dynamometer parasitic losses). The inventive test device is able to operate in this way because parasitic losses are primarily speed and temperature dependent and can be accurately characterized by other means (i.e., dynamometer coast-down tests that measure dynamometer speed decay when unloaded at a higher speed). As a consequence of using the dynamometers own load cell to measure load, the inventive test device can also test dynamometers using multiple load measuring devices or load cells and multiple power absorbers, as is the case with non-physically coupled dynamometers. One such device is described in U.S. Pat. No. 5,101,660 which is hereby incorporated by reference in its entirety.
The following is a typical equation used to characterize dynamometer parasitic losses across a speed range after they are calculated at several speeds from data obtained by performing unloaded coastdowns (See e.g. EPA-AA-RSPD-IM-96-2, xe2x80x9cAcceleration Simulation Mode Test Procedures, Emissions Standards, Quality Control Requirements, and Equipment Specificationsxe2x80x9d, Section 85.4(b)(2), July 1996, and EPA420-R-00-007, xe2x80x9cIM240 and Evap Technical Guidancexe2x80x9d, April 2000, Section 85.2226(a)(4), both available at www.epa.gov/oms/im.htm, which are hereby incorporated by reference):
Parasitic Losses=(Axc3x97v2)+(Bxc3x97v)+C
Where:
A, B, C are regression coefficients
v=dynamometer speed
Equations for determining parasitic losses such as the above equation are familiar to people experienced in dynamometry.
Once the dynamometer parasitic losses are characterized, the target dynamometer load as a function of speed and time can be calculated according to the following equation derived from the EPA IM240 technical guidance (See e.g., EPA420-R-00-007, xe2x80x9cIM240 and Evap Technical Guidancexe2x80x9d, Section 85.2226(a)(2), April 2000).
PAU LOAD (measured)=INERTIA LOAD (function of acceleration)+ROAD LOAD (function of speed)xe2x88x92BASE INERTIA LOAD (function of acceleration)xe2x88x92DYNAMOMETER PARASITICS (function of speed)xe2x88x92TIRE ROLLER FRICTIONAL LOAD (function of speed)
Where:
PAU LOAD=Load applied by the Power Absorption Unit, required to accurately simulate the vehicle characteristics.
INERTIA LOAD=Load required to accelerate the vehicle""s mass at the applicable acceleration rate. This factor is derived from the simple physics of F=ma where f=force, m=vehicle mass and a=acceleration at the given time.
ROAD LOAD=Load required to offset the frictional forces encountered by the vehicle at the applicable speed. Road load is typically defined using a polynomial equation detailed in the EPA IM240 specification (See e.g., EPA420-R-00-007, xe2x80x9cIM240 and Evap Technical Guidancexe2x80x9d, Appendix H, April 2000).
BASE INERTIA LOAD=Load required to accelerate the dynamometer""s rotating mass at the applicable acceleration rate.
DYNAMOMETER PARASITICS=Load required to offset the dynamometer""s friction at the applicable speed. See the aforementioned equation detailing parasitic losses.
TIRE ROLLER FRICTIONAL LOAD=Load required to offset the frictional forces at the tire/roller interface at the applicable speed. Tire losses are typically quantified according to the xe2x80x9cgeneric tire/roll lossxe2x80x9d (GTRL) equations in the EPA IM240 specification (See e.g., EPA420-R-00-007, xe2x80x9cIM240 and Evap Technical Guidancexe2x80x9d, Appendix I, April 2000).
As with the parasitic loss determination equations, the equation for dynamometer loading and the variables included within are familiar to people experienced in dynamometry.
Since each of these factors, aside from PAU load, is a function of speed or acceleration and can be quantified independently, speed and acceleration are the only pieces of information necessary to predict PAU loading during a transient simulation. Speed is preferably recorded once per second (other intervals may also be chosen), while the load may be recorded as the median of multiple readings preferably over one second (other intervals may again be selected). Speed is recorded independently of the dynamometer control system with an optical tachometer, fifth wheel or similar speed measurement device.
Once the speed and load measurements have been recorded by the inventive test device, the theoretical load versus the actual load can be compared to determine the dynamometer accuracy using a variety of different analytical methods. One possible method examines overall error. The overall error method compares the sum of the second-by-second applied load to the sum of the second-by-second calculated load at the end of the test. Another possible analytical method reviews instantaneous error during a test. Instantaneous error is calculated by comparing the measured load to the actual load at each second of the test. A variation of the instantaneous method involves taking short-duration moving averages (e.g., 5 seconds) and comparing those averages throughout the test. Criteria defining the range of acceptable performance will vary depending on the vehicle test program requirements. For example, the Massachusetts Department of Environmental Protection has an instantaneous horsepower simulation percent error limit of 5%, with readings averaged over a 5 second interval (xe2x80x9cMASS99 Specifications, Version 1.3R1xe2x80x9d, Section 2.5.4.b.2, Aug. 21, 2000, Massachusetts Department of Environmental Protection, One Winter Street, Boston, Mass., 02108). Other comparable analytical criteria may also be employed and are within the skill of people experienced in dynamometry.